Recently, in the casae where on FM broadcast is received by an FM receiver mounted in a car, output of the reception is hampered by multipath noise produced by so-called FM multipath disturbance due to the fact that a direct wave of the FM broadcast and reflected waves reflected by buildings or mountains are inputted simultaneously in a receiving antenna. Therefore it is very offensive to the ear and the quality of the reception is impaired. For this reason, in order to reduce the multipath noise and to improve the quality of the reception, generally a method described below is adopted.
FIG. 4 is a circuit diagram of a prior art FM multipath noise reduction circuit used in an FM receiver.
The receiving system in such a receiver consists of an FM IF circuit 1 amplifying high frequency signals of e.g. 10.7 MHz and an FM multiplexer ( MPX ) circuit 2 for demodulating the IF output into FM stereo signals, i.e. left and right side signals.
The multipath noise reduction circuit 3 is connected to this receiving system. This circuit 3 consists of a multipath noise detection circuit 4, in which the S meter voltage V.sub.s corresponding to the received electric field intensity (voltage for driving a meter usually disposed in a receiver and indicating the signal intensity) of the IF circuit 1 is inputted and which detects multipath noise therein, a negative rectification circuit 5 rectifying the signal outputted by the circuit 4 so as to obtain a negative voltage, and an adder 6, which adds the output voltage of this circuit 5 and the meter voltage V.sub.s and applies the sum of them to the FM MPX circuit 2. By such a construction the stereo separation of the FM MPX circuit 2 is controlled by utilizing the S meter voltage, which is proportional to the electric field intensity included in the IF stage and further the S/N ratio is improved by attenuating the signal at the high frequency region.
That is, paying attention to the fact that the S meter voltage is decreased instantaneously, which gives rise to harmonic wave component noise, when FM multipath noise is produced, harmonic wave components are detected by the multipath noise detection circuit 4. When this detected voltage is rectified by the negative rectification circuit 5 and added to the original S meter voltage by means of the adder 6, the voltage S.sub.NC outputted by the adder 6 works so as to reduce the S meter voltage, corresponding to the FM multipath noise. In this case the stereo separation of the FM stereo demodulator is varied by using the S meter voltage as the control voltage. The stereo separation is worsened with decreasing value of this voltage and the separation becomes nearer to the monaural state with increasing FM multipath noise. Furthermore, since the attenuation at the high frequency region increases, the S/N ratio increases and thus the quality of the reception is improved.
With progressing fabrication of electronic devices using ICs one-chip ICs (integrated circuits) 10 of block construction, e.g. as indicated in FIG. 5, are more widely used as the multipath reduction circuit 3 enclosed by a broken line in FIG. 4.
In FIG. 5 the multipath reduction circuit 10 is composed of a control output system consisting of a first level shift circuit 11, a charge/discharge circuit 12, a condenser 13 for charge/discharge connected with the charge/discharge circuit 12 and a second level shift circuit 14, and a multipath noise detection system treating the high frequency components (hereinbelow called alternating current voltage V.sub.AC) due to rapid decrease of the S meter voltage and controlling the charge/discharge circuit 12.
The multipath noise detection system is composed of a V - I converter 15 for V - I (voltage - current) converting the alternating current voltage V.sub.AC, a current doubler circuit 16, which doubles the converted current and outputs it, a charge/discharge control circuit 17 controlling the charge/discharge circuit 12 on the basis of the output of the circuit 16, a charging current generation circuit 18 for supplying charging current to the charge/discharge circuit 12, and a constant voltage circuit 19 for supplying a stabilized voltage to each of the circuits. Further a condenser C.sub.1 is disposed for cutting the direct current component V.sub.DC of the S meter voltage V.sub.S.
Now the operation of the multipath noise reduction circuit 10 in FIG. 5 will be explained.
When the S meter voltage V.sub.S is inputted, the level shift circuits 11 and 14 output an output voltage V.sub.O (DC OUT), which has a certain relationship to he direct current component V .sub.I (DC IN), as indicated in FIG. 6. A charge/discharge condenser 13 (C.sub.2) is charged to a constant voltage, when no multipath noise is produced.
The alternating voltage V.sub.AC in the S meter voltage is detected through the V - I converter 15 and the current doubler circuit 16. When it exceeds a certain threshold level A, as indicated in FIG. 7, the charge/discharge condenser 13 is discharged by the charge/discharge control circuit 17, depending on the threshold level, and the output voltage V.sub.O of the second level shift circuit 14 decreases. When the FM multipath noise disappears, the charge/discharge condenser 13 is again charged. FIG. 8 indicates this charge/discharge operation.
In FIG. 8 T.sub.d represents the decay time of the control output voltage S.sub.NC, when FM multipath noise exceeding a certain threshold level is applied to AC IN and it is set by the external charge/discharge condenser C.sub.2 and a resistor R.sub.O not shown in the figure, which is built-in in the IC 10. EQU Td=C.sub.2.R.sub.o ( 1)
Further T.sub.r represents the rise time, when the FM multipath noise disappears and the control output voltage returns to its original value, and it can be determined by the external charge/dischare condenser C.sub.2 and an external resistor R.sub.1. This external resistor R.sub.1 determines the charging current of the charge/discharge condenser C.sub.2 and T.sub.r is determined by the following formula; EQU Tr=C2.Vc/I (2)
I : charging current set by external resistor R.sub.1
C.sub.2 : external charge/discharge condenser
V.sub.c : voltage between charge/discharge condenser C.sub.2 and GND
Consequently, according to this method the S meter voltage is controlled by utilizing charge/ discharge characteristics of the external charge/discharge condenser C.sub.2.
The voltage applied to the external condenser C.sub.2 is (V.sub.I +.DELTA.V), because the direct current V.sub.I applied to DC IN is shifted up by .DELTA.V by the first level shift circuit 11 and shifted down by .DELTA.V by the second level shift circuit 14. Further, when DC IN is O V, it is at a voltage of .DELTA.V.
Therefore the usual rise time T.sub.r ' and the rise time T.sub.r " when the power source is switched-on can be represented by the following formulas; ##EQU1##